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Not all green Hydroelectric power production is seen an important part of the renewable energy mix, but a new study challenges its green credentials in tropical regions.

Globally hydroelectricity makes up 16 per cent of the total amount of electricity produced each year, and the number of new hydroelectric power projects are steadily increasing, particularly in developing countries.

Despite concerns about their effects on wildlife, displaced local populations and changes to river flows, hydroelectricity is considered by many to be a sustainable source of energy generation.

A study published in this week's Nature Geoscience analysed the emissions of carbon dioxide and methane from 85 hydroelectric reservoirs around the world and found the carbon output was highest in young reservoirs in the lower latitudes.

Carbon dioxide and methane emissions are caused by the decomposition of flooded vegetation and soil organic matter, especially during the period immediately following the flooding of a river valley. Later, organic matter continues to be added to the reservoir from inflowing rivers, algal production and regrowth of vegetation along the fringes.

The study, which was led by Nathan Barros from the Federal University of Juiz de Fora in Brazil found that all reservoirs from boreal latitudes to the tropics were a source of methane, with 88 per cent also emitting CO2.

They calculated around 48 megatons of carbon per year are emitted as CO2 and a further 3 megatons of carbon are emitted as methane from the reservoir surface.

The highest emissions came from the tropics, especially the Amazon region, where the bottom waters and sediments tend to be oxygen-free, leading to anaerobic decomposition that contributes significantly to methane production.

Small, but not insignificant

Barros and colleagues' findings correspond to about 4 per cent of global carbon emissions from inland waters. They say this is smaller than previous estimates, because they were based on more limited data. They add the research doesn't account for factors such as the loss of the potential carbon sink in the flooded area or sedimentation-related offsets.

"What the study is really saying is that while the greenhouse gas emissions associated with hydroelectric reservoirs are relatively small in the scheme of things, they are not insignificant and need to be taken into account when planning any new hydroelectric projects, especially schemes being planned in tropical areas or the Amazon," says Dr David Harries, an adjunct professor with the School of Electrical, Electronic and Computer Engineering at the University of Western Australia.

Harries, who was not involved in the research, says the study doesn't take into account emissions from irrigation reservoirs, such as Lake Argyle in north-western Australia.

"More importantly, the study does not include the greenhouse gas emissions that result from building the roads, the dams and clearing vegetation," he says.

"What all of this is saying that a full greenhouse gas life cycle analysis should be undertaken for any proposal to build a large reservoir, whether it is a reservoir to be used for hydroelectricity, irrigation, or town water supply."

Australia's carbon output

Barries says the size of hydroelectricity schemes in Australia is small compared to the rest of the world, and given their location, would be relatively small contributors of greenhouse gases.

"The largest hydroelectric impoundments in Australia are in the Snowy Mountains and in Tasmania," he says. "These are temperate alpine areas and so the greenhouse gas emissions per square kilometre of surface area would be less than hydroelectric schemes in hotter climates such as Brazil, Indonesia or Malaysia."

"[Also] the age of these reservoirs ranges from about 50 to 40 years, so their greenhouse gas emissions would have declined appreciably from when they were first built."

"Having said that it is worth noting that it is possible for older and mid-latitude hydroelectric reservoirs to still have high methane emissions. Lake Wohlen in Switzerland, for example, is 90 years old, but a recent study found methane emissions from the lake are still high."

Harries says he is unaware any plans to build large-scale hydroelectric reservoirs in Australia, although he adds there is a proposal to construct a large-scale freshwater impoundment project known as the Ord River Stage 2.

"[In light of this study] as the Kimberley is a tropical area, the greenhouse gas emissions associated with both of those would need to be looked at," he says.